In general, wrapping materials must have various characteristics depending on the type, shape, weight, etc. of the article to be wrapped therewith. Particularly, wrapping materials for use in making bags must have superior physical characteristics, that is, have excellent tear strength, tensile strength, burst strength, impact strength and so on.
Wrapping materials having such excellent physical strength have been proposed, for example, in Japanese Utility Model Application Laid-open Nos. 133631/77 (corresponding to U.S. Pat. No. 4,147,291) and 21168/80 (corresponding to U.S. Pat. No. 4,258,848 and G.B. Pat. No. 2,026,942A). These known wrapping materials are so-called cross-lamainated film wrapping materials having the layer structure schematically illustrated in FIG. 1. More specifically, two uniaxially-stretched high density polyethylene films 1 and 2 are disposed so that their stretching axes form an angle of 45.degree. to 90.degree. with each other, and they are bonded together through an adhesive layer 3. As the adhesive layer 3, a thermoplastic resin, such as low density polyethylene, is usually employed. In some cases, however, films 1 and 2 may be heat-sealed at a suitable heating temperature of about 180.degree. C., for example, without the application of the adhesive layer 3.
These cross-laminated film wrapping materials have satisfactory strength in any of longitudinal, lateral and oblique directions because the stretching axes intersect each other. Thus, compared with conventional wrapping materials, for example, composite laminate films comprising a low density polyethylene film and paper, an aluminum foil or the like, they have greatly improved physical strength.
It has been found, however, that the physical strength of the cross-laminated film wrapping material varies depending on the thickness of the adhesive layer 3 and that the physical strength decreases with increasing thickness. The reason for this is believed to be in bonding films 1 and 2 together with adhesive layer having a thickness which is beyond the critical value, they are completely united into a body and act as substantially one film although they are disposed in such a manner that their stretching axes intersect each other.
Therefore, it is necessary to minimize the thickness of the adhesive layer 3. However, if the thickness of the adhesive layer 3 is reduced excessively, breakage of the adhesive layer 3 will occur. This leads to a peelingoff of the films 1 and 2 at the part where breakage of the adhesive layer 3 occurs.
On the other hand, in the method where films 1 and 2 are heat-sealed, heat-shrinkage of the stretched film occurs at the areas heated. This results in the formation of wrinkles which are not desirable from the standpoint of quality.